The present invention relates generally to a variable length link for a vehicle mounted power lift system adapted to connect a working implement to the vehicle and specifically relates to a variable length link for a three-point hitch for an agricultural tractor.
A typical agricultural tractor includes what is commonly referred to as a three-point hitch which is adapted to connect an implement to the tractor and permit the tractor to raise and lower the implement. The typical three-point hitch includes a pair of draft links, a pair of lift links, a pair of lift arms, and a compression or center link. The draft links have their forward ends pivotally connected to the tractor and the rear ends are adapted to be connected to an implement. The lift links interconnect the draft links and the lift arms so that upon rotation of the lift arms, the rear ends of the draft links are raised and lowered. The center link has it forward end pivotally connected to the tractor and its rear end is adapted to be connected to the implement. In the typical three-point hitch, at least one of the lift links is adjustable in length to permit lateral leveling of an attached implement and the center link is usually adjustable in length to permit fore-and-aft leveling of an attached implement. The lift links and center link are normally manually adjustable.
In order to reduce operator effort and permit adjustments on-the-go, there have been suggestions to provide a power operated device to adjust the length of various links on a tractor three-point hitch. For example, U.S. Pat. No. 3,572,761 discloses an adjustable length lift link which incorporates a hydraulic cylinder and piston arrangement for varying the length of the link. U.S. Pat. No. 3,850,450 discloses a center link which includes a hydraulic motor for rotating a screw which varies the length of the link. U.S. Pat. Nos. 3,561,789 and 3,572,763 both disclose an adjustable length center link which may be in the form of a hydraulic cylinder and piston arrangement or an electric motor arrangement. The use of a hydraulic cylinder and piston arrangement to vary the length of a link in a three-point hitch has not been entirely satisfactory since the length of the link could vary due to leakage across the piston. Adjustable length links which used a screw have not been entirely satisfactory since the power requirements have been very large, particularly if adjustments were to be made while the implement is in use.